1. Technical Field
The present invention relates to an image sensing device, comprising a detection means of a fluorescent lamp flicker under an illumination of a flickering light source such as the fluorescent lamp, in an XY-addressed-scanning type image sensor, for example, a CMOS image sensor.
2. Background Art
In a case where an object is imaged by a video-camera under an illumination of a fluorescent lamp directly lit on a commercial power source, a periodical brightness change, so called a fluorescent lamp flicker, is seen in a video signal of imaging output, due to a difference between a brightness variation period of the fluorescent lamp (an invert of a double the commercial power source frequency) and a vertical synchronization period of the video-camera, unless an exposure time is set to the brightness variation period of the fluorescent lamp multiplied by an integer. Particularly, in a case of an XY-addressed-scanning type image sensor such as a CMOS image sensor being used, the flicker is observed on an imaging plane as striped patterns due to a vertically periodical variation of a brightness intensity level or a hue, because the exposure timing is different between horizontal lines.
Here, strictly speaking, with the XY-addressed-scanning type image sensor such as the CMOS image sensor being used, the exposure timing per each pixel is sequentially shifted by a period of a read-out clock (pixel clock) from one pixel to its adjacent one, on each horizontal scanning line on the imaging plane. Even in case to assume that all of in-line pixels have the simultaneous exposure timing, no substantial problem is caused, although the exposure timings in all pixels are different. This is because a scanning time for a horizontal line is much shorter than the intensity variation period of the fluorescent lamp.
As a technique and method, to remove the flicker portion from the imaging plane signal, a shutter correction method to make corrections based on a relation between a shutter speed and a flicker level, and a gain correction method to put an inverted waveform of a detected flicker waveform onto the video signal as a correction gain, are principally known.
As an example of the shutter correction method, JP Publication H04-373365 describes a method to integrate a video signal for a one-field period using an integration circuit, to store the signal for the one-field period at shortest, then to sequentially compare a current input signal with a one-field previous input signal from the integration circuit, subsequently to detect the flicker by determining an existence of a regularity, and to control the shutter speed.